1. Field of Invention
This invention relates to the prevention of backfeed of power into an alternating current (AC) power source when supplying a load from alternate sources of AC power.
2. Description of Related Art
Various systems exist for selectively supplying AC power to a load from first or second AC sources. Typically such systems may have input terminals comprising line and neutral terminals for receiving AC power from the first AC source and output terminals comprising output line and neutral terminals for receiving AC power from the second source and for supplying AC power to the load. The first AC source is often an AC mains supply provided by an electric utility company and the second AC source often involves an inverter or inverter/charger device.
Typically a transfer switch is connected between the input terminals and the output terminals and is operable to selectively connect the input terminals to the output terminals or to isolate the input terminals from the output terminals. Such a transfer switch may be provided by a relay, for example, having a coil energized by the first AC source and configured such that while AC power is available from the first AC source the load receives power from the first AC source. Where the second AC source is an inverter/charger device, circuitry in the device detects that AC power is being supplied by the first source and shuts down the device or causes it to assume a charge mode in which batteries connected to the device are charged using current drawn from the first AC source.
In the event that a voltage or current anomaly occurs at the input terminals, perhaps due to a fault in the electric utility, the relay acting as the transfer switch may momentarily drop out while current is being supplied to the load, which can cause the relay contacts to become welded together. Alternatively, a fault may occur within the relay itself or in the system as a whole that causes the relay to stay in a connection mode, in which the input terminals are connected to the output terminals, even though the relay may be unenergized.
When a transfer switch relay undesirably remains in the above mentioned connection mode and the inverter/charger device attempts to supply power to the load, backfeed of power into the first AC source can occur. This can be hazardous. In many areas, electrical codes require that should backfeed occur, the source of the backfeed must be shut off within a certain period of time, such as 2 seconds, to avoid perturbing the electric utility system.
Prior methods for sensing backfeed have involved employing a contact in a single pole double throw arrangement whereby a first contact of this arrangement is connected to the line terminal of the first AC source, a second contact is connected to sensing circuitry and a common contact is connected to the output line terminal. When the relay is energized the common contact is connected to the first contact and when the relay is unenergized, the common contact is connected to the second contact. This scheme Is used with the assumption that if the first contact is welded to the common contact, and it is known that the relay is not energized, the second contact will not be connected to the common terminal. This condition is sensed and, in response, a signal is produced to cause the second AC source to shutdown. This solution has limited flexibility as a special relay contact arrangement must be used. This scheme is also solely directed to detecting whether or not the transfer switch relay has malfunctioned and is incapable of detecting other anomalous conditions such as a floating ground condition, where the neutral output terminal is at a different potential than a ground terminal.